Electron beam welding of two dissimilar metals

ABSTRACT

In the welding of two metals which are substantially dissimilar with respect to at least one pure metal component such that in normal practice a zone of embrittlement occurs in the weld, a ductile weld is obtained by means of an electron beam applied along a line closely adjacent but spaced from the center line of the weld with the energy input of the electron beam being controlled so that the concentration of the pure metal component in the finished weld remains substantially constant across the width of the weld except for a very fine zone of transition at the edge of the weld wherein it changes to equal the concentration of that component in one of the two metals.

United States Patent ELECTRON BEAM WELDING OF TWO DISSIMILAR METALS 2Claims, 9 Drawing Figs.

U.S.Cl 219/121 Int. Cl B23k 15/00 FieldotSearch 219/121, 121158, 137, 69

Primary Examiner.l. V. Truhe Assistant ExaminerR. E. ONeillAttorney-Burgess, Dinklage and Sprung ABSTRACT: In the welding of twometals which are substantially dissimilar with respect to at least onepure metal com-,

ponent such that in normal practice a zone of embrittlement occurs inthe weld, a ductile weld is obtained by means of an electron beamapplied along a line closely adjacent but spaced from the center line ofthe weld with the energy input of the electron beam, being controlled sothat the concentration of the pure metal component in the finished weldremains substantially constant across the width of the weld except for avery fine zone of transition at the edge of the weld wherein it changesto equal the concentration of that component in one of the two metals.

PATENTEUFEB 20: 3560100 sum 1 0F 6 Fig. I

I 2 lnconel s CrNi I8 8 oin/ans:

Wa ner fi e r26 Relic/Slur). Ofr Ran Man PATENTEDFEB 2:91: 3560.700

sum 2 or 6 Inconl 600 ATENIFU FEB PISII SHEU 3 U? 6' Fig.3

I I CrNi l8 8- PATENTEDFEBPIQTI I aigso jog ELECTRON BEAM WELDING OF TWODISSIMILAR METALS The invention relates to a welding procedure for thereduction or elimination of brittle intermetallic phase(s) which arenormally produced in the welding seam between metallic materials andwhich adversely affect the stability of the welding joint.

These materials include, e.g., the metals titanium, zirconium, niobium,vanadium used in nuclear technology, aviation and spacecraft industryand also special cases in conventional technology, as well as theiralloys, e.g., zircaloy, V Ti, which have to be welded to conventionalmaterials, such as high temperature and corrosion steels, aluminum,copper etc.

Due to their excellent corrosion resistance against boiling water inpressure-tube reactors and on account of their small neutron absorptionzirconium and its alloys are employed in reactor engineering as canningmaterials for fuel rods. The same applies to titanium, vanadium andtheir alloys which are preferrably used structural materials in reactortechnology because of their favorable corrosion properties and theirhigh strength.

The welding of two alloys which are only little different from eachothere.g. the welding of different steels-is already a complicatedbusiness and only possible by the use of appropriate filling materialsand by thermal treatment procedures. Much less transparent and, hence,much more complicated are the problems encountered when welding verydissimilar materials, e.g. steels to titanium, zirconium, niobium,vanadium or their alloys, since even in the case of simple systems,e.g., binary systems, and assuming the state diagrams of thesemulticomponent systems as known, the formation of mixed crystals in themelting zone cannot be deducted from the state diagrams due to the verydissimilar cooling conditions depending on the type of welding methodemployed.

Though the formation of mixed crystals can be influenced to a certainextent with the aid of conventional welding methods by using fillingmaterials, this requires in almost every case an appropriate thennaltreatment which is, however, disliked in reactor engineering for severalreasons.

It is the task of the present invention to influence the mixing ratiosexisting in the welding zone in a way as to allow for a sound welding ofeven dissimilar materials susceptible to the formation of brittleintermetallic phases in the melting zone, without the use of fillingmaterials and by observing the required purity levels as well asavoiding prior and subsequent thermal treatments, so that flawless,ductile welding joints capable of carrying heavy loads can be achieved.

It was studied by the use of several welding methods to what extent theconcentration development in the welding seam and hence the stabilitybehavior can be influenced.

The first testing materials used for obtaining reliable date data on themixture ratios in the melted-on zone were two alloys which can be weldedsatisfactorily to each other, but which are very different with regardto at least one alloy component.

From this aspect the two alloys CrNi I8 8 (about 70 percent iron) andInconel 600 (about 5 percent iron) are selected. These materials arebutt-welded to each other as 0.5 mm. thick sheets, using the WIG-methodas well as the electron beam method of welding. The concentrationdevelopment of iron and nickel in the melting zone is determined by amicroprobe. The weldings according to the WIG-method in the absence offilling materials led, in general, to the observation that even byvarying the welding parameters current" and welding speed" awell-defined concentration development could not be achieved. As is seenfrom FIG. 1, the iron concentration measured over the whole width of thewelding seam, varies very arbitrarily between the lowest iron content incase of lnconel 600 and the highest iron content in the case of CrNi l88; approximately in the center of the melting zone it returns to theoriginal value of 5 percent, while it visibly decline after another risebefore it approaches the concentration of 70 percent on the side of CrNi18 8 steel. At slower welding speeds of less than 2 mm/s. with thecurrent kept constant a plateau with smaller variations is reached atapproximately 35 percent iron content. It was not possible to vary thelevel of the plateau in a reproducible manner.

By the use of the electron beam method the possibility of influencingthe mixing and cooling conditions seemed to increase because of thedensity of energy enhanced by the factor 10 the focused application ofenergy and the large range of variations of the welding parameters,e.g., acceleration tension, beam current, welding speed, beam movementand duration of beam application (pulsed electron beam).

Taking the same testing materials CrNi l8 8 and lnconel 600 the attemptwas made with the electron-beam welding method to influence thedevelopment of concentration. A pulsed electron beam was particularlysuited to control in a unique way and with the possibility ofreproduction the development of concentration as a function of the pointof impingement of the electron beam.

FIG. 2 shows the different points of impingement of the electron beam.The dash-dotted line 5 represents the joint of the two materials CrNi l88 l and lnconel 600 2 to be welded to each other. In the test weldingthe electron beam was applied parallel to the joint 5 at a distancerepresenting a multiple of 0.9 mm. and the concentration development wasmeasured over the welding seam width. On the lnconel 600 side 2 thewelding seam was displaced by 0. 0.18 mm. 3 and by 0.09 mm. 4 parallelto the joint. Subsequently, a welding was made with the point ofimpingement of the electron beam situated within the joint 5. On theside of the CrNi l8 8 steel I the points of impingement of the electronbeam were displaced by 0.09 mm. 6, by 0.18 mm. 7 and by 0.27 mm. 8parallel to the joint.

FIG. 3 shows the iron-concentration development 9 plotted against thewelding-seam width in case of an electron-beam welding, as it isobserved following the displacement of the point of impingement by 0.18mm. 3 on the side of lnconel 600 2, as well as the iron-concentrationdevelopment I0 observed following the displacement of the point ofimpingement into the joint 5 and the iron-concentration development IIobserved following the displacement of the point of impingement by 0.27mm. 8 on the side of the CrNi I8 8 steel 1. The nickel-concentrationdevelopment takes always a complementary course to that of theiron-concentration development, but has not been plotted on the FIG. forreasons of cleamess. s means the width of the welding seam.

In FIG. 4 the Fe-concentration curves are represented in linear formover the welding-seam width s, i.e., without considering the variationsin concentration and different welding seam widths, for the differentdistances 38 of the points of impingement from the joint 5 of thematerials CrNi l8 8 steel 1 and Inconel 600 2 to be welded andrepresented in FIG. 2. The FIG. shows that practically any concentrationdevelopment can be reproducibly achieved. Both the level of theindividual plateaus l2 and the position and degree of concentrationleap's 13, 14 can be varied. In order to achieve more clearness in therepresentation of the course of concentration curves, the concentrationleaps I3 and 14 were laterally transposed. The variations inconcentration depend primarily on the welding speed and the manner ofbeam conduction. By these preliminary studies with model materials whichdo not form intermetallic phases, but which allow for an excellentexamination of the concentration development over the welding-seam widthdue to the homogeneous alloy components available in variousconcentrations, the theoretical fundamentals for welding dissimilarmaterials which are susceptible to form brittle intermetallic phases inthe melt, could be found.

The invention solves the task set at the beginning of this descriptionby preferrably influencing with the aid of a charge carrier beam theconcentration development of the material components over thewelding-seam width s through the welding parameters-intensity, type,duration of effect and above all, point of application of the weldingenergy-so that a constant concentration of material components isobtained over the major part of the welding-seam width s, which liesconsiderably above or below the concentration of the intermetal licphase(s) and that the one of the intermetallic phase is traversed in anextremely narrow zone of welding-seam width A s with the help of aconcentration leap. The narrow zone A s in which brittle intermetallicphases still occur. is less than 10 '15, as results from hardnessmeasurements and structure examinations.

The method according to the invention uses a charge carrier beam, sinceit allows for a concentrated application of energy and an exact settingof all parameters, especially the point of impingement.

The method according to the invention offers particular advantages incase that an energy-dosed pulsed electron beam taking into account thethermal properties of the materials to be welded will be used accordingto the relation Furthermore, the welding of zirconium or its alloys toother metals, such as iron and austenitic steels becomes possible withthe aid of the method according to the invention.

Vanadium or its alloys could also be welded to austenitic steels oraluminum etc. by using the method according to the invention.

In the case of niobium or its alloys it was also possible with themethod according to the invention to weld them to iron and nickel ortheir alloys.

The following table lists for some material combinations the bendingangles obtained, which are a measure for the ductility of the weldingjoint, as a function of the welding parameters (acceleration tension,intensity of beam current, welding speed, periodand impulse length) andthe lateral displacement of the electron beam from the joint:

Welding parameter Point of impinge- Bending Material combination I v LLi merit of the beam angle, [kv 1 [11111.] [rnm.ls.] msl [ms.] degreeSheet thickness 0,5 mm. AI-Cu 105 2, 40 1 0,4 0, 1 in Al 180 105 2,0 401 0, 4 0, 1 in Fe----. 180 130 1,4 20 1 0,4 0, 15 in A1 180 105 2, 0 4O1 0, 4 Independent. 180 105 2,0 40 1 0,4 -do 180 105 2,0 40 1 0,4 d0 180105 3,3 40 0,36 0,15 0,2in Cu.--.. 180 130 1, 5 20 1 0,4 0, in Fe 180105 2,4 40 1 0,4 0 1 in Fe-.. 180 130 1, 3 1 0, 4 0, 15 in F6" 90 1300,9 20 0, 36 0, 5 0, 15 in 1%.. 45 130 1,3 20 1 0,4 0,15inV- 120 120 1,2 15 0, 1 in CrNi 18 8 180 130 1, 7 20 1 0,4 0, 1 in CrNi 18 8..." 110130 1, 9 20 1 0, 4 0. 75 in CrNi 18 8...- 110 130 1, 3 20 1 0, 4 0, 15in CrNi 18 8...- 130 1,9 20 0, 36 0, 15 0, 15 in CrNi 18 8.... 105 2,440 1 0,4 0, 1 in CrNl18 8 180 105 2,4 40 1 0,4 0, 15 in GrNi 18-- 180130 1,3 20 1 0,4 0, 2 in CrNl 18 8-0.. 15 130 1, 3 20 1 0,4 0, 1 180 1301, 3 20 1 o, 4 o, 1 180 130 1, 3 20 1 0, 4 0, 1 180 Ni-V 105 2, 4 40 10, 4 0, 1 180 Income! 600Nb... 130 1,7 20 1 0,4 0,1 180 Sheet thickness,2 mm.

Ni-Al .1 130 3, 0 20 1 0, 4 0, 1 180 Zr-Al 1. 130 3,0 20 1, 0 0,4 0, 1180 5 and in case that the point of impingement of this electron ratiobetween period length and impulse length U acceleration tension in Voltv welding speed in cm/s However, in specific cases a continuous beam canbe used instead of a pulsed electron beam, the ratio between periodlength and impulse length L,,/Li being 1.

The above relation is the decisive prerequisite for the concentrationleap being exactly the same in each depth of materials, especially formaterial thicknesses up to 5 mm.

The method according to the iiivention can be used to weld any materialcombination in which brittle intermetallic phases occur. In these casesthe bending angles only depend on the ductility of the mixed crystal.

Thus, the method according to the invention allowed, e.g., to weldtitanium or its alloys to other construction materials, such as iron andsteel, and above all austenitic steels or copper, etc.

The foregoing table demonstrates the decisive importance attached to thepoint of impingement of the electron beam and the remaining weldingparameters. The extent to which 0 this point of impingement must bedisplaced laterally to the joint into the material can be very easilyfound. by a few preliminary studies in which the point of impingement isshifted into the material at a distance of some 0.05-0.3 mm. from thejoint and the ductility of the welding joint is determined with the helpof the maximum attainable bending angle.

With the aid of the method according to the invention flawless andductile welding joints can even be achieved in the most difficult cases,so that hardness values in the melted-on zone do not in part exceedthose of the two starting materials and bending angles up to and moreare possible.

In the following paragraphs the invention shall be explained in moredetail with the aid of FIGS. 5 through 9 and taking as examples thematerial combinations V 10 Ti-Cr Ni 18 8. The thickness of sheetspecimens was always 0.5 mm.

FIG. 5 shows the concentration development of iron over the welding seamwidth in case of a welding with optimum welding parameters.

FIGS. 6 and 7 show the hardness development over the welding scam; inthe first case the welding according to the invention was carried outwithout a lateral displacement of the electron beam, whilst in thesecond case it was carried out in the absence of any displacement FIG. 8shows a metallographic micrograph of the welding specimen performedunder the method according to the invention with optimum conditions ofwelding parameters.

joint which has become molten during the welding or at least Ichemically transfon'ned. This FIG. shows that the iron content :which isnought in the material V Ti-apart from impuri ties-jerkily increases inthe boundary region of thewe lding seam s in a zone of only few m widthto reacha Fe-concentration of 70 percent; it also shows that thisconcentration of 70 percent remains subsequently constant over .thewhole about 0.27 mm. wide welding seam s and suddenly changes over tothe Fe-concentration in Cr Ni I8 8 steel. Since for these materialconcentrations the intermetallic phases and/or brittle mixed crystalsoccur in Fe-concentration zones below 70 percent and this zone can bebridged by a concentration leap with the method according to theinvention, or in other words, since these intermetallic phases and/orbrittle mixed crystals can be contracted to a zone of few um width,ductile and flawless welding joints can be achieved. The welding datafor the performance of this welding joint are: U 105 kV; l= 2.4 mA; v 40mm/s; L lms; L,- 0.4 ms; the point of impingement of the electron beambeing displaced laterally by 0.15 mm. into the Cr Ni 18 8 material.

In FIG. 6 the development of microhardness along the weld ing seam isplotted for a welding in which the electron beam was focused to thejoint between V Ti and Cr Ni I8 8 contrary to the method according tothe invention. On the ordinate are plotted the hardness values obtainedwith the Vickers method for a load of 0.050 kp/mm." along the measuringsection which is diagonal to the welding seam. Measurement is startedwith the V 10 Ti material where hardness values of about 200 kp/mm.occur. In the zone s melted-on during the welding, the hardness valuesreach prohibitively high values of about 750 lap/mm. due to theformation of mixed crystals, whilst they decrease to 200 kp/mm. outsidethe welding seam s in the Cr Ni 18 8 zone. With the hardness valuesachieved in this case, the welding joint is so brittle that it breakswhen it is charged with relative small loads.

As a comparison, FIG. 7 shows the development of microhardness plottedover the welding seam, as it is observed under the method according tothe invention following the lateral displacement of the point ofimpingement of the electron beam by 0.15 mm. from the joint into the CrNi 18 8 material. Apart from lateral displacement, the same weldingparameters, i.e., U 105 kV; I 2.4 mA; v mm/s; L =l ms; L, 0.4 ms. wereused for both weldings. The course of the hardness curve shows that thehardness values in the welding seam s do not exceed the hardness valuesin the basic materials V 10 Ti and Cr Ni 18 8 steel. As a consequence,very ductile welding seams are obtained.

FIG. 8 shows a metallographic micrograph of a welding seam ahundredtimes enlarged which is produced under the method according tothe invention. For the welding the same welding parameters were used asdescribed in the FIGS. 5 and 7, i.e., U I05 kV; I 2.4 mA; v=40 mm/s; L Ims; L,- 0.4 ms; lateral displacement of the point of impingement of theelectron beam towards the joint by 0.15 mm. into the Cr Ni 18 8material. The measuring points arranged in a horizontal line constitutethe microhardness impressions with a load of 50 pond/mm I-I 0.050 kplmmcaused by the evaluation procedure. On the left-hand side of the pictureof the V 10 Ti-material can be recognized which is separated from thezone of the welding melt S (center of the picture) by a series ofvertical black lines representing the contracted intermetallic phase andbeing situated between the 5. and 6. measuring point on the left side.Next to the welding melt S lies the material Cr Ni 18 8 in which aresituated the 6 right measuring points. The ductile welding joint isachieved by this enormous contraction of the intermetallic phase.

FIG. 9 represents a section showing the contracted intermetallic phases2 50 times enlarged. It can be seen that the intermetallic phases areinterrupted several times.

Thematerial combinations entered in the table could not besatisfactorily welded with the aid of conventional methods. Due toextremely high hardness values they broke or produced cracks immediatelyafter melding. Only in cases wherethe conditions of the method accordingto the invention are observed, welding seams could be obtained which aresatisfactory in every respect.

' We claim:

l. A welding method for the reduction of the brittle intermetallic phasenormally produced in a welded seam between two dissimilar metals whichphase adversely affects the stabili ty of the welded joint, comprisingapplying a beam of charged particles to cause fusion in the area of theweld and controlling the application of the same, the welding parametersbeing controlled according to the following equation:

(Watt'em) I Li Uv 7 10 wherein:

I= beam current in Amp., L,, period length in milliseconds, L,-= impulselength in milliseconds,

keying ratio,

U acceleration voltage in volt, and

v welding velocity in cm/sec, and the position of the line ofimpingement of said beam being in the range of 0.07 to 0.2 mm. away fromthe center of the joint, such that there is a constant concentration ofthe metal components over a major portion of the width of the weld whichis substantially different from the normal concentration thereof in saidintermetallic phase, but intermediate of the concentrations thereof ineach of said two dissimilar metals, whereby the zone of saidintermetallic phase is relatively quite small and the concentrationchange of the metal components therein is quite precipitous.

2. In the welding of two dissimilar metals each of which in the area ofwelding has an initial thickness less than about 5 mm., said twodissimilar metals with respect to a single pure metal component havingsubstantially different amounts of said metal component and said weldingbeing effected by progressively heating and fusing of the joint alongthe line of weld by impingement thereon of an electron beam, theimprovement resulting in a minimization of the zone of the phase changeacross which the concentration of said pure metallic component changesfrom the higher value as it exists in one of said two dissimilar metalsto the lower value as it exists in the other, comprising:

impinging said electron beam along a line parallel to the center line ofsaid weld and spaced in the range of 0.07 to 0.2 mm. therefrom, andcontrolling the energy introduced into said weld according to:

(Watt-cm.) I Li U v 7 10 wherein:

I= beam current in Amp.,

L, period length in milliseconds,

L,= impulse length in milliseconds,

U beam voltage in volts, and

v welding velocity in cm/sec.; the completed weld having,

with respect to said pure metal component, a substantially constantconcentration thereof intermediate of the concentration thereof in eachof said dissimilar metals, across substantially the entire width of thezone of fusion of the metal with said zone of phase change being at theedge thereof within which said substantially constant concentrationchanges precipitously to equal the concentration of said pure metalcomponent in one of said two dissimilar metals, and said completed weldhaving a hardness value at any intermediate point in said zone of fusionthat does not exceed the hardness values of either of said twodissimilar metals.

2 33 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3.560.700 L-W wajeidelum M It is certified that error appears intheabove-identified paten and that said Letters Patent are hereby correctedas shown below:

C01. 2, line 7, change "10 4 co'--10 -10 C01. 3, line 2, change "one" to"zone";

Col. 4, the table, under "sheet thiokness 0,5 m

the 19th entry, change "Cr-Ni 18-8-V10Ti" to -CrNi 18-8-V1OTi-- I Col. 4the table, under the heading "point of impingement," the 19th entry;ohange "0.15 in CrNi to --0.15 in CrNi 1s a--. Col. 5 line 4 change "aFe-concentrationf' to o --a specimeng line 11, after m" insert--(micromet Signed and sealed this 9th day of May 1972.

(SEAL) Attest:

EDWARD M.FLETGHER, 'JR ROBERT GQTTSQHALK IAjztesting OfficerCommissioner of Patents

1. A welding method for the reduction of the brittle intermetallic phasenormally produced in a welded seam between two dissimilar metals whichphase adversely affects the stability of the welded joint, comprisingapplying a beam of charged particles to cause fusion in the area of theweld and controlling the application of the same, the welding parametersbeing controlled according to the following equation: wherein: I beamcurrent in Amp., Lp period length in milliseconds, Li impulse length inmilliseconds, keying ratio, U acceleration voltage in volt, and vwelding velocity in cm/sec, and the position of the line of impingementof said beam being in the range of 0.07 to 0.2 mm. away from the centerof the joint, such that there is a constant concentration of the metalcomponents over a major portion of the width of the weld which issubstantially different from the normal concentration thereof in saidintermetallic phase, but intermediate of the concentrations thereof ineach of said two dissimilar metals, whereby the zone of saidintermetallic phase is relatively quite small and the concentrationchange of the metal components therein is quite precipitous.
 2. In thewelding of two dissimilar metals each of which in the area of weldinghas an initial thickness less than about 5 mm., said two dissimilarmetals with respect to a single pure metal component havingsubstantially different amounts of said metal component and said weldingbeing effected by progressively heating and fusing of the joint alongthe line of weld by impingement thereon of an electron beam, theimprovement resulting in a minimization of the zone of the phase changeacross which the concentration of said pure metallic component changesfrom the higher value as it exists in one of said two dissimilar metalsto the lower value as it exists in the other, comprising: impinging saidelectron beam along a line parallel to the center line of said weld andspaced in the range of 0.07 to 0.2 mm. therefrom, and controlling theenergy introduced into said weld according to: wherein: I beam currentin Amp., Lp period length in milliseconds, Li impulse length inmilliseconds, U beam voltage in volts, and v welding velocity incm/sec.; the completed weld having, with respect to said pure metalcomponent, a substantially constant concentration thereof intermediateof the concentration thereof in each of said dissimilar metals, acrosssubstantially the entire width of the zone of fusion of the metal withsaid zone of phase change being at the edge thereof within which saidsubstantially constant concentration changes precipitously to equal theconcentration of said pure metal component in one of said two dissimilarmetals, and said completed weld having a hardness value at anyintermediate point in said zone of fusion that does not exceed thehardness values of either of said two dissimilar metals.